New bounds on Axion-Like Particles in the Ultraviolet from Legacy Data

We use legacy data from the Hubble Space Telescope (HST) and the International Ultraviolet Explorer (IUE) to search for a spectral line from the spontaneous decay of axion-like particle (ALP) dark matter. The HST data consist of blank sky observations taken with the Faint Object Spectrograph in the 165--240~nm wavelength range, while the IUE data consist of observations of the Virgo Cluster obtained with the long- and short-wavelength spectrographs, covering 195--325~nm and 123--200~nm, respectively. We set a 95\% C.L. upper limit on the ALP--photon coupling $g_{a\gamma} \lesssim 10^{-11}~\mathrm{GeV}^{-1}$ across the whole probed ALP mass range. Notably, we rule out values of $g_{a\gamma}$ above $2.3 \times 10^{-12}~\mathrm{GeV}^{-1}$ for ALP masses between 12.4 and 14.5\,eV, improving upon previous limits by a factor of seven.